Refactor logger.New to accept FieldLogger

While doing this I've also cleaned up the code a little bit. Let me know
if you think the otherwise.

Closes: #823

log/logger.go

@@ -31,21 +31,22 @@ func NewNullLogger() *Logger {
 }
 
 // New creates a new logger.
-func New(logger *logrus.Logger, iterID string) *Logger {
-	var defLogger bool
+func New(logger logrus.FieldLogger, iterID string) *Logger {
+	var ll *logrus.Logger
 	if logger == nil {
-		defLogger = true
-		logger = logrus.New()
+		ll = logrus.New()
+		ll.Warn("Logger", "no logger supplied, using default")
 	}
-	l := &Logger{
-		Logger: logger,
-		iterID: iterID,
-	}
-	if defLogger {
-		l.Warnf("Logger", "no logger supplied, using default")
+	var ok bool
+	if ll, ok = logger.(*logrus.Logger); !ok {
+		// panic so that we know when to upgrade. see issue #818.
+		panic("logger is not a logrus.Logger")
 	}
 
-	return l
+	return &Logger{
+		Logger: ll,
+		iterID: iterID,
+	}
 }
 
 // Tracef logs a trace message.